Revert 'Workaround TSan false positive about clear_hdr_marks/realloc race'

Revert 'Workaround TSan false positive about clear_hdr_marks/realloc race'

This reverts commit 20468dd131d82be7422aea3c7383ec9201452fce.

Because it is not a false positive.  And we are lying to the compiler
about the value not changing asynchronously, which is never good.
The proper solution could be use of atomic accesses (even unordered)
on the size field.