[profile] Delete zero-size dummy sections

They were added so that if no metadata section is present,
`__start_llvm_prf_*` references would not cause "undefined symbol"
errors.  By switching to undefined weak symbols in D96936, the dummy
sections are not needed.

This patch is also needed to work around
https://sourceware.org/bugzilla/show_bug.cgi?id=27490

Differential Revision: https://reviews.llvm.org/D97648