gdb/dwarf: remove support for DWP v1

gdb/dwarf: remove support for DWP v1

The GCC wiki [1] says that version 1 of the DWP format "was an
experimental version that is no longer supported".  That comment was
written in 2013.  From what I understand, DWP version 1 was not widely
adopted.  Unlike version 2 and version 5 (which is part of DWARF 5).

I propose to get rid of v1 support, which allows removing a bit of code,
making things a bit easier when reasoning about DWARF code.

[1] https://gcc.gnu.org/wiki/DebugFissionDWP

Change-Id: Ie46b55662e63b7f341b7827330154ec4ab5af9e5
Approved-By: Tom Tromey <tom@tromey.com>