[docs] Remove unsupported references to ExtraSource variable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156857 91177308-0d34-0410-b5e6-96231b3b80d8

Makefile.common

@@ -30,8 +30,7 @@
 #
 # 4. Source - If specified, this sets the source code filenames.  If this
 #    is not set, it defaults to be all of the .cpp, .c, .y, and .l files
-#    in the current directory.  Also, if you want to build files in addition
-#    to the local files, you can use the ExtraSource variable
+#    in the current directory.
 #
 # 5. SourceDir - If specified, this specifies a directory that the source files
 #    are in, if they are not in the current directory.  This should include a

docs/Projects.html

@@ -402,13 +402,6 @@ <h3>
 <div>
 
 <dl>
-  <dt>ExtraSource
-  <dd>
-  This variable contains a space separated list of extra source
-  files that need to be built.  It is useful for including the
-  output of Lex and Yacc programs.
-  <p>
-
   <dt>CFLAGS
   <dt>CPPFLAGS
   <dd>