bfd/elfnn-aarch64.c: Cleanup a couple of TLS functions

Add an assert and remove an unused line of code.

bfd/ChangeLog:

2014-06-04  Will Newton  <will.newton@linaro.org>

        * elfnn-aarch64.c (tpoff_base): Make test of tls_sec
	being non-NULL into an assert.
	(elfNN_aarch64_tls_relax): Remove unused code.