[CMAKE] Install DLL to bin/ by default

Summary:
The shared libraries are installed by CMake to lib/ by default, with
the exception of windows DLLs which are installed under bin/.
This behavior was changed in D4229 to reflect the location of the
delivered files from our release.

However it appears that the windows Gitian build is searching for the
DLL files in the bin/ directory, before copying it to lib/. This
workaround is then unnecessary as it is handled by Gitian, and can be
simply removed.

Test Plan:
Cross-build for Windows:

ninja install

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4403