clean up BambaDecoderLayer __init__
Author: garrett361
Pull request: yes

# What does this PR do?

Very minor cleanup of the `BambaDecoderLayer.__init__` method. Avoids introducing a potentially confusing, but essentially unused, `num_experts` variable.

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Comments:
CC @fabianlim 

Changed this because I was initially confused by the code when reading it quickly.

Closing; will consolidate with other changes into a separate PR.