DebugInfo: Don't include DW_AT_inline on each abstract definition multiple times.

When I refactored this in r208636 I accidentally caused this to be added
multiple times to each abstract subprogram (not accounting for the
deduplicating effect of the InlinedSubprogramDIEs set).

This got better in r208798 when the abstract definitions got the
attribute added to them at construction time, but still had the
redundant copies introduced in r208636.

This commit removes those excess DW_AT_inlines and relies solely on the
insertion in r208798.