configure: Remove --enable-incompatible-libav-abi from the help output

As it does not work anymore since 655b6dcb34

Found-by: carl
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -108,7 +108,6 @@ Configuration options:
   --enable-gray            enable full grayscale support (slower color)
   --disable-swscale-alpha  disable alpha channel support in swscale
   --disable-all            disable building components, libraries and programs
-  --enable-incompatible-libav-abi enable incompatible Libav fork ABI [no]
   --enable-raise-major     increase major version numbers in sonames [no]
 
 Program options: