stop adding EUMM to build requires

[karenetheridge]

EUMM is not needed to run 'make' - there is no reason why MI should be populating build-requires at all.

[haarg]

EUMM is needed by various parts of the Makefile, and it adds itself to build requires. There isn't really any harm in doing this, although it also isn't much help because it's too late for the prereq to be satisfied.

[miyagawa]

Actually, MakeMaker puts itself to "build" too, even if MI stops doing that. (I just tested with a plain Makefile.PL with WriteMakefile() call)

➜ cat Makefile.PL 
use ExtUtils::MakeMaker;
WriteMakefile(NAME => "Foo");
➜  perl Makefile.PL 
Generating a Unix-style Makefile
Writing Makefile for Foo
Writing MYMETA.yml and MYMETA.json
➜  grep -3 build MYMETA.json
      ]
   },
   "prereqs" : {
      "build" : {
         "requires" : {
            "ExtUtils::MakeMaker" : "0"
         }