cmake: reset USING_MOLD_LINKER when WITH_MOLD is off

USING_MOLD_LINKER is an INTERNAL cache entry set only inside the
if(WITH_MOLD) branch, so it survives a reconfigure. Turning WITH_MOLD off
in an existing build directory leaves it TRUE, while the -fuse-ld=mold
flags (plain variables appended to CMAKE_*_LINKER_FLAGS) are dropped.
The build then links with BFD but keeps taking the mold code paths in
Distutils.cmake, CephChecks.cmake and src/crimson/osd/CMakeLists.txt.

Clear it in an else() branch so it always tracks WITH_MOLD.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>