Revert "[CMake] Provide Findzstd module"

This reverts commit 8490598.

This breaks running llvm tests:

llvm-lit: /home/npopov/repos/llvm-project/llvm/utils/lit/lit/TestingConfig.py:138: fatal: unable to parse config file '/home/npopov/repos/llvm-project/build/test/lit.site.cfg.py', traceback: Traceback (most recent call last):
  File "/home/npopov/repos/llvm-project/llvm/utils/lit/lit/TestingConfig.py", line 127, in load_from_path
    exec(compile(data, path, 'exec'), cfg_globals, None)
  File "/home/npopov/repos/llvm-project/build/test/lit.site.cfg.py", line 48, in <module>
    config.have_zstd = FALSE
NameError: name 'FALSE' is not defined