realtek_mst_i2c_spi.c: Remove dead code

Turns out the MST likely doesn't need these so-called defaults
to be written for the purposes of spi flashing.

BUG=b:152558985,b:148745673
BRANCH=none
TEST=builds

Change-Id: Ieb938cf0805b22692d61db23795208c9be962b60
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/41124


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sam McNally <sammc@google.com>