[fpga,ci] Remove stub MMI file generation

Description

After #30543 was merged, the Vivado MMI file generation is now not necessary on master, since we no longer use updatemem to splice the ROM and OTP images into synthesized bitstreams.

To reduce the scope of that PR whilst still enabling further HW iteration (such as increasing the ROM size), that PR simply replaced the MMI file generation logic with some simple code to generate a stub/dummy file with some garbage contents. Since this is unused, we should remove generation of this dummy file, remove its appearance in the bitstream cache (may be incompatible with the old schema and so may need a bitstream cache schema change with an appropriate version bump), and remove Bazel targets that refer to the MMI file on the master branch.

Start with hw/top_earlgrey/util/vivado_hook_write_bitstream_pre.tcl, then search the repository for MMI references in bitstream cache definitions and Bazel targets. Remove the unused stub generation and references, updating the cache schema version if required; done means master no longer generates or caches the dummy MMI file.