auxdisplay: img-ascii-lcd: kconfig: Remove MIPS_SEAD3 reference

auxdisplay: img-ascii-lcd: kconfig: Remove MIPS_SEAD3 reference

Commit 3f5f0a4475e1 ("MIPS: generic: Convert SEAD-3 to a generic board")
removed the MIPS_SEAD3 symbol and moved the setting of IMG_ASCII_LCD to
the board-sead-3.config defconfig file, but IMG_ASCII_LCD still
references the removed MIPS_SEAD3 symbol.

Remove the reference.

Discovered with the
https://github.com/ulfalizer/Kconfiglib/blob/master/examples/list_undefined.py
script.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>