[BOLT][DWARF] Delete DW_AT_low_pc when converting to ranges

Now that we have new DWARF Rewriter we can remove DW_AT_low_pc when converting
DW_AT_low_pc/DW_AT_high_pc to DW_AT_ranges. Which closer follows DWARF spec.
Leaving CU DW_AT_low_pc in place. Reading the spec I think it's needed.

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D156957