[clang][modules] Remove preloaded SLocEntries from PCM files (#66962)

This commit removes the list of SLocEntry offsets to preload eagerly
from PCM files. Commit introducing this functionality (258ae54a) doesn't
clarify why this would be more performant than the lazy approach used
regularly.

Currently, the only SLocEntry the reader is supposed to preload is the
predefines buffer, but in my experience, it's not actually referenced in
most modules, so the time spent deserializing its SLocEntry is wasted.
This is especially noticeable in the dependency scanner, where this
change brings 4.56% speedup on my benchmark.