gdb: remove deprecated versions 7.2.x and 7.3.x

Gdb versions 7.2.x and 7.3.x have been deprecated since 2013.02 and thus can
be removed in 2014.02.
For legacy handling, version 7.5.x is automatically selected.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>