TrigInterfaces Cleanup, master branch (2020.04.08.)

Did a technical cleanup in the TrigInterfaces package... I was "inspired" to do so while working on !31633 (merged). And realised that in a debug build (for platform x86_64-centos7-gcc8-dbg) it may take >20 minutes to link the libTrigInterfaces.so library. Which, by the way, doesn't hold any useful code.

While at it, I did a few different things:

• Simplified the CMake configuration of the package to its bare essentials.
  • Removed the usage of atlas_depends_on_subdirs(...). (Hopefully this will not cause build problems in other packages, but it very well may...)
  • Checked all declared dependencies on the main library and the tests, and kept only the ones that are actually being used by the current code.
  • Removed the build of the useless TrigInterfaces library. Only keeping TrigInterfacesLib.
• Removed all unnecessary files from the package.
• Removed the executable flag from all the source files of the package.

Unfortunately the build may not succeed, as dependent packages may not like these CMake configuration simplifications. If so, I'll fix the clients of TrigInterfaces in this same MR as well. 😉