cmake: stop trying to build the reftable and xdiff libraries

cmake: stop trying to build the reftable and xdiff libraries

In the `en/make-libgit-a` topic branch, more precisely in the commits
f3b4c89d59f1 (make: delete REFTABLE_LIB, add reftable to LIB_OBJS,
2025-10-02) and cf680cdb9543 (make: delete XDIFF_LIB, add xdiff to
LIB_OBJS, 2025-10-02), the strategy to build three static libraries was
rethought, and instead only one static library is now built.

This is good.

However, the CMake definition was not changed accordingly, and now
CMake-based builds fail thusly:

  [...]
  Generating hook-list.h
  CMake Error at CMakeLists.txt:122 (string):
    string sub-command REPLACE requires at least four arguments.
  Call Stack (most recent call first):
    CMakeLists.txt:711 (parse_makefile_for_sources)

  CMake Error at CMakeLists.txt:122 (string):
    string sub-command REPLACE requires at least four arguments.
  Call Stack (most recent call first):
    CMakeLists.txt:717 (parse_makefile_for_sources)

  -- Configuring incomplete, errors occurred!

Fix that by removing the parts that expect the reftable and xdiff
objects to be defined separately in the Makefile, still.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>