[yaml2obj] - Cleanup and simplify the code. NFCI.

The current code has the following problems:
`initSymtabSectionHeader` and `initStrtabSectionHeader` method
names saying us they are going to initialize the section headers.
Though for a few cases sh_flags field is initialized outside of them.
It does not look clean. This patch moves initialization of the
sh_flags inside these methods.

Also, it removes an excessive variable, what together with the above
change hopefully makes the code a bit more readable.

llvm-svn: 359443