gdb: remove versions 7.4 and 7.5

We already default to 7.6 for all architectures (except AVR32, ARC and
Microblaze that have their specific versions), and we have added 7.7
recently.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>