Remove MMX code path in lexer

Host systems with only MMX and no SSE2 should be really rare now.
Let's remove the MMX code path to keep the number of custom
implementations the same.

The SSE2 code path is also somewhat dubious now (nearly everything
should have SSE4 4.2 which is >15 years old now), but the SSE2
code path is used as fallback for others and also apparently
Solaris uses it due to tool chain deficiencies.

libcpp/ChangeLog:

	* lex.cc (search_line_mmx): Remove function.
	(init_vectorized_lexer): Remove search_line_mmx.