Remove GCC 4.7 from drone

The C++11 support required for Boost.SharedPtr is not met

Author: Flamefire

.drone.jsonnet

@@ -98,13 +98,6 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
 };
 
 [
-    linux_pipeline(
-        "Linux 16.04 GCC 4.7",
-        "cppalliance/droneubuntu1604:1",
-        { TOOLSET: 'gcc', COMPILER: 'g++-4.7', CXXSTD: '0x' },
-        "g++-4.7",
-    ),
-
     linux_pipeline(
         "Linux 16.04 GCC 4.8",
         "cppalliance/droneubuntu1604:1",