sparc: drop -mlra

The sparc port gained LRA support in r7-5076-gf99bd883fb0d05 and has
defaulted to LRA since r7-5642-g70a6dbe7e37e69.

Let's finish the transition by dropping -mlra entirely.

Tested on sparc64-unknown-linux-gnu with no regressions.

gcc/ChangeLog:
	PR target/113952
	* config/sparc/sparc.cc (sparc_lra_p): Delete.
	(TARGET_LRA_P): Ditto.
	(sparc_option_override): Don't use MASK_LRA.
	* config/sparc/sparc.md (disabled,enabled): Drop lra attribute.
	* config/sparc/sparc.opt: Delete -mlra.
	* config/sparc/sparc.opt.urls: Ditto.
	* doc/invoke.texi (SPARC options): Drop -mlra and -mno-lra.