cmake-server: Revert "Support codemodel filegroups for INTERFACE_SOURCES"
Revert !2282 (merged). The changes activate code paths not meant to be used with interface libraries. Another approach will be needed to expose this information later.
This revert has to be done by hand because the code in question has been changed somewhat since the changes were made, and was also factored out to another source file.
Fixes: #18463 (closed)