gdb/dwarf2: remove erroneous comment in open_and_init_dwo_file

When writing commit 28f15782adab ("gdb/dwarf: read multiple .debug_info.dwo
sections"), I initially thought that the gcc behavior of producing multiple
.debug_info.dwo sections was a bug (it is not).  I updated the commit
message, but it looks like this comment stayed.  Remove it, since it can
be misleading.

Change-Id: I027712d44b778e836f41afbfafab993da02726ef
Approved-By: Tom Tromey <tom@tromey.com>