Fix f18 install method (#13123)

* Install method is broken and not needed. Built-in install method works fine. Just delete.

* Change version to master.

var/spack/repos/builtin/packages/f18/package.py

@@ -14,10 +14,6 @@ class F18(CMakePackage):
 
     git      = "https://github.com/flang-compiler/f18"
 
-    version('develop', branch='master')
+    version('master', branch='master')
 
-    depends_on('llvm@6.0.0+clang', when='@develop')
-
-    def install(self, spec, prefix):
-        mkdirp(prefix.bin)
-        install("spack-build/tools/f18/bin/f18", prefix.bin)
+    depends_on('llvm@6.0.0+clang', when='@master')