rtlwifi: rtl8192de: fw.c: Remove unused function

Remove the function rtl92d_set_fw_pwrmode_cmd() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>