rtlwifi: btcoexist: Remove CamelCase variable

File halbt_precomp.h contains the only CamelCase value, namely
GetDefaultAdapter, but that macro is never used, thus it is deleted.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>