daemon: Remove btrfs_noversion build flag

btrfs_noversion was added in d7c37b5a28de6e7c0a9270815c092a45d8d7fef7
for distributions that did not have the `btrfs/version.h` header file.

Seeing how all of the distributions we currently support do have the
`btrfs/version.h` file we should probably just remove this build flag
altogether.

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>

daemon/graphdriver/btrfs/version.go

@@ -1,4 +1,4 @@
-// +build linux,!btrfs_noversion
+// +build linux
 
 package btrfs // import "github.com/docker/docker/daemon/graphdriver/btrfs"
 

daemon/graphdriver/btrfs/version_none.go

@@ -1,14 +0,0 @@
-// +build linux,btrfs_noversion
-
-package btrfs // import "github.com/docker/docker/daemon/graphdriver/btrfs"
-
-// TODO(vbatts) remove this work-around once supported linux distros are on
-// btrfs utilities of >= 3.16.1
-
-func btrfsBuildVersion() string {
-	return "-"
-}
-
-func btrfsLibVersion() int {
-	return -1
-}

daemon/graphdriver/btrfs/version_test.go

@@ -1,4 +1,4 @@
-// +build linux,!btrfs_noversion
+// +build linux
 
 package btrfs // import "github.com/docker/docker/daemon/graphdriver/btrfs"
 

hack/make.sh

@@ -115,14 +115,6 @@ elif ${PKG_CONFIG} 'libsystemd-journal' 2> /dev/null ; then
 	DOCKER_BUILDTAGS+=" journald journald_compat"
 fi
 
-# test whether "btrfs/version.h" exists and apply btrfs_noversion appropriately
-if \
-	command -v gcc &> /dev/null \
-	&& ! gcc -E - -o /dev/null &> /dev/null <<<'#include <btrfs/version.h>' \
-; then
-	DOCKER_BUILDTAGS+=' btrfs_noversion'
-fi
-
 # test whether "libdevmapper.h" is new enough to support deferred remove
 # functionality. We favour libdm_dlsym_deferred_remove over
 # libdm_no_deferred_remove in dynamic cases because the binary could be shipped