gdb: remove tic6x .dat files

The tix6x gdbserver port was modified to use target descriptions in
commit 506fe5f49967 ("Change tic6x target descriptions").  The old
regformats .dat files were kept as a way to make sure the new target
descriptions matched the old register decsriptions.  I think by now it's
not necessary to keep the .dat files.

I don't have a way to build-test this though.

Change-Id: Ia90b5ae6381234c6e95555201d3e65ed9be880ea
Approved-By: Tom Tromey <tom@tromey.com>