Remove Safe mode

Summary:

• rpc: Move RPC_FORBIDDEN_BY_SAFE_MODE code to reserved section

Although this code is no longer ever sent back after removing safe mode,
it would be unwise to remove it from the header.

For one, it would be bad to accidentally re-use the number.

Also some API documentation / bindings are directly generated from the .h
file - this is why the "Aliases for backward compatibility" are there. We don't
want to break code that relies on this error code existing, even if it's never
generated.

So keep it around but move it to a reserved section.

• Remove "rpc" category from GetWarnings

No longer used after removing safe mode.

This function can likely be simplified more, but I'll leave that
for later to make this easy to review.

This is a backport of Core PR13090

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2820