gdb, gdbsupport: use DISABLE_COPY_AND_ASSIGN in a few spots

Replace some explicitly deleted copy constructor and copy assignment
operator with the DISABLE_COPY_AND_ASSIGN macro for consistency
with the rest of the codebase.

Change-Id: If3fe2c4d7b3cb4530eace86d589116f805c7656f
Approved-By: Tom Tromey <tom@tromey.com>