staging: rtl8192e: Remove unused macro dm_tx_bb_gain_idx_to_amplify

Remove unused macro.

Signed-off-by: Teddy Engel <engel.teddy@gmail.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/5ee3df2b02a84e6aeb2d1c4903cb018161d78b1b.1718309120.git.engel.teddy@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>