[ThinLTO] Disable partial sample profile scaling by default

As pointed out in
https://discourse.llvm.org/t/undeterministic-thin-index-file/69985, the
block count added to distributed ThinLTO index files breaks incremental
builds on ThinLTO - if any linked file has a different number of BBs,
then the accumulated sum placed in the index files will change, causing
all ThinLTO backend compiles to be redone.

This was only used for partial sample profiles, and was therefore
removed for other cases (3adc6e03).

Subsequent testing did not show a performance effect of disabling this
feature even for partial sample profiles. Therefore, switch the default
to false. If this does not cause a noticeable performance degradation
after the default flip, we can remove this support completely.

Differential Revision: https://reviews.llvm.org/D151249