cmake-server: Revert "Support codemodel filegroups for INTERFACE_SOURCES"

Revert commit v3.13.0-rc1~144^2 (cmake-server: Support codemodel
filegroups for INTERFACE_SOURCES, 2018-08-10).  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