tcg: Remove softmmu code_gen_buffer fixed address

tcg: Remove softmmu code_gen_buffer fixed address

The commentary talks about "in concert with the addresses
assigned in the relevant linker script", except there is no
linker script for softmmu, nor has there been for some time.

(Do not confuse the user-only linker script editing that was
removed in the previous patch, because user-only does not
use this code_gen_buffer allocation method.)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>