Remove DWARF3 testing of test/DebugInfo/X86/gnu-public-names.ll

This was added to test that DW_AT_GNU_pubnames used sec_offset in DWARF4
and data4 in DWARF3 and below. Since then we've updated
DW_AT_GNU_pubnames to be a flag, rather than a section offset anyway.

Granted this still differs between DWARF 3 and DWARF 4
(FORM_flag_present versun FORM_flag) but it doesn't seem worthwhile
testing that codepath again here. It's covered adequately in many other
test cases.

And while I'm here, don't hardcode the byte size of the compile unit -
it's not relevant to this test and just makes it brittle if/when
anything changes in the way this CU is emitted.