accel/ivpu: Remove legacy firmware name

We are now using NPU IP generation based FW names instead of platform
code names, so mtl_vpu.bin can be removed.

Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240214081305.290108-3-jacek.lawrynowicz@linux.intel.com