refactor(cmake): remove obsolete local module overrides

Drop three legacy CMake module files that are no longer referenced
anywhere in the build system:
  - src/cmake/FindSSE.cmake

SSE/AVX detection is handled by Eigen and the compiler directly.
Compiler flag checking is now done via the built-in CheckCompilerFlag
module (CMake 3.19+) in CodeCoverage.cmake.

Author: Sylvain Maziere