[lldb/COFF] Remove strtab zeroing hack

Summary:
This code (recently responsible for a unaligned access sanitizer
failure) claims that the string table offset zero should result in an
empty string.

I cannot find any mention of this detail in the Microsoft COFF
documentation, and the llvm COFF parser also does not handle offset zero
specially. This code was introduced in 0076e715, which also does not go
into specifics, citing "various bugfixes".

Given that this is obviously a hack, and does not cause tests to fail, I
think we should just delete it.

Reviewers: amccarth, markmentovai

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D83881