bsd-user: Simplify the implementation of execve

bsd-user: Simplify the implementation of execve

This removes the logic which prepends the emulator to each call to
execve and fexecve. This is not necessary with the existing
imgact_binmisc support and it avoids the need to install the emulator
binary into jail environments when using 'binmiscctl --pre-open'.

Signed-off-by: Doug Rabson <dfr@rabson.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>