Remove unreferenced local variables:

Increase signal to noise in appveyor build output by reducing the MSVC warning count from 12 to 4. 12 is the number of MSVC warnings under our current appveyor setup assuming the changes in #14086 are also implemented.

This makes it easier to spot errors or more important warnings in the verbose appveyor output. MSVC warnings are good, so having access to them in a noise free way (read: without trivial warnings) via appveyor without having to use Windows is really valuable.

See https://github.com/bitcoin/bitcoin/pull/14086#issuecomment-416610313 plus discussion for context.

Before:

```
c:\projects\bitcoin\src\script\script.cpp(272): warning C4018: '>': signed/unsigned mismatch [C:\projects\bitcoin\build_msvc\libbitcoinconsensus\libbitcoinconsensus.vcxproj]
c:\projects\bitcoin\src\rest.cpp(467): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\libbitcoin_server\libbitcoin_server.vcxproj]
c:\projects\bitcoin\src\test\allocator_tests.cpp(147): warning C4312: 'reinterpret_cast': conversion from 'int' to 'void *' of greater size [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\coins_tests.cpp(511): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\coins_tests.cpp(524): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\coins_tests.cpp(722): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\coins_tests.cpp(783): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\crypto_tests.cpp(535): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?) [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\dbwrapper_tests.cpp(265): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\net_tests.cpp(118): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\net_tests.cpp(151): warning C4101: 'e': unreferenced local variable [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\scheduler_tests.cpp(57): warning C4305: 'argument': truncation from 'int' to 'bool' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
```

After:

```
c:\projects\bitcoin\src\script\script.cpp(272): warning C4018: '>': signed/unsigned mismatch [C:\projects\bitcoin\build_msvc\libbitcoinconsensus\libbitcoinconsensus.vcxproj]
c:\projects\bitcoin\src\test\allocator_tests.cpp(147): warning C4312: 'reinterpret_cast': conversion from 'int' to 'void *' of greater size [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\crypto_tests.cpp(535): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?) [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
c:\projects\bitcoin\src\test\scheduler_tests.cpp(57): warning C4305: 'argument': truncation from 'int' to 'bool' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
```

ninja check