packages: Mark 'replacement' as an "innate" field.

Suggested by Mark H Weaver
at <https://lists.gnu.org/archive/html/guix-devel/2017-06/msg00355.html>.

* guix/packages.scm (<package>)[replacement]: Mark as "innate".
* gnu/packages/base.scm (glibc-2.25-patched, glibc-2.24)
(glibc-2.23, glibc-2.22, glibc-2.21, glibc-locales): Remove
'replacement' field, which was set to #f.
* gnu/packages/commencement.scm (perl-boot0): Likewise.
* gnu/packages/fontutils.scm (graphite2/fixed): Likewise.
* gnu/packages/ghostscript.scm (ghostscript/fixed): Likewise.
* gnu/packages/gnupg.scm (libgcrypt-1.7.8): Likewise.
* gnu/packages/guile.scm (guile-2.0/fixed, guile-2.2): Likewise.
* gnu/packages/icu4c.scm (icu4c/fixed): Likewise.
* gnu/packages/image.scm (libpng-apng): Likewise.
* gnu/packages/make-bootstrap.scm (%guile-static): Likewise.
* gnu/packages/pcre.scm (pcre/fixed): Likewise.
* gnu/packages/perl.scm (perl/fixed): Likewise.
* gnu/packages/ruby.scm (ruby-2.3, ruby-2.2, ruby-2.1)
(ruby-1.8): Likewise.
* gnu/packages/tls.scm (gnutls-3.5.13, gnutls/guile-2.2): Likewise.
* gnu/packages/xml.scm (expat-2.2.1): Likewise.

gnu/packages/ruby.scm

@@ -106,7 +106,6 @@ a focus on simplicity and productivity.")
   (package
     (inherit ruby)
     (version "2.3.4")
-    (replacement #f)
     (source
      (origin
        (method url-fetch)
@@ -124,7 +123,6 @@ a focus on simplicity and productivity.")
 
 (define-public ruby-2.2
   (package (inherit ruby)
-    (replacement #f)
     (version "2.2.7")
     (source
      (origin
@@ -138,7 +136,6 @@ a focus on simplicity and productivity.")
 
 (define-public ruby-2.1
   (package (inherit ruby)
-    (replacement #f)
     (version "2.1.10")
     (source
      (origin
@@ -172,7 +169,6 @@ a focus on simplicity and productivity.")
 
 (define-public ruby-1.8
   (package (inherit ruby)
-    (replacement #f)
     (version "1.8.7-p374")
     (source
      (origin