Revert "[lldb][test] Allow '.c' files to be used as shell tests (llvm#161520)"

This reverts commit e6fd6d9.

Author: adrian-prantl

lldb/test/Shell/lit.cfg.py

@@ -25,7 +25,7 @@
 
 # suffixes: A list of file extensions to treat as test files. This is overriden
 # by individual lit.local.cfg files in the test subdirectories.
-config.suffixes = [".test", ".cpp", ".s", ".m", ".ll", ".c"]
+config.suffixes = [".test", ".cpp", ".s", ".m"]
 
 # excludes: A list of directories to exclude from the testsuite. The 'Inputs'
 # subdirectories contain auxiliary inputs for various tests in their parent