gdb/doc: trim trailing whitespaces

My editor "accidentally" removed all trailing whitespaces from
gdb.texinfo while doing a change.  That was mostly just an annoyance
but to avoid it happening again, I suggest removing them for good.

I look at the difference in the output of "make html".  The new output
has some trailing whitespaces removed, but none of them seems to cause a
semantic difference.  Not sure about other formats like info or pdf
though.

Change-Id: I3f349b28c581af69703365fea07e7b93614c987c
Approved-By: Eli Zaretskii <eliz@gnu.org>