configure: Remove deprecated incompatible_fork_abi option

It's been deprecated since ffmpeg 2.0

Signed-off-by: James Almer <jamrial@gmail.com>
Analysis of the usage of the switches by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -106,7 +106,6 @@ Configuration options:
   --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-incompatible-fork-abi  enable incompatible Libav fork ABI (deprecated) [no]
   --enable-raise-major     increase major version numbers in sonames [no]
 
 Program options:
@@ -1447,7 +1446,6 @@ CONFIG_LIST="
     $SUBSYSTEM_LIST
     fontconfig
     incompatible_libav_abi
-    incompatible_fork_abi
     memalign_hack
     memory_poisoning
     neon_clobber_test
@@ -1581,7 +1579,6 @@ HAVE_LIST_PUB="
     bigendian
     fast_unaligned
     incompatible_libav_abi
-    incompatible_fork_abi
 "
 
 HEADERS_LIST="
@@ -4139,10 +4136,6 @@ EOF
     exit 1;
 fi
 
-# backward compatibility layer for incompatible_libav/fork_abi
-enabled incompatible_fork_abi  && enable incompatible_libav_abi
-enabled incompatible_libav_abi && enable incompatible_fork_abi
-
 die_license_disabled() {
     enabled $1 || { enabled $2 && die "$2 is $1 and --enable-$1 is not specified."; }
 }