firefox: drop unused version

component/firefox/buildout.cfg

@@ -71,11 +71,6 @@ part = ${firefox-60:location}
 wrapper-name = firefox-52
 part = ${firefox-52:location}
 
-[firefox-wrapper-51]
-<= firefox-wrapper
-wrapper-name = firefox-51
-part = ${firefox-51:location}
-
 [firefox-default-fonts-conf]
 recipe = slapos.recipe.template:jinja2
 template = ${template-fonts-conf:output}
@@ -124,13 +119,6 @@ version = 52.9.0esr
 i686-md5sum = 9aa18888b7812670208490609d75c9bc
 x86_64-md5sum = 9336d70f45070c743d08e5473b783a7a
 
-[firefox-51]
-<= firefox-download
-version = 51.0.1
-i686-md5sum = 9a5b67e9d759a1e4df004294a24b2b43
-x86_64-md5sum = bd93f2652d1d90d59ae462439a93c85f
-
-
 [firefox-download]
 recipe = slapos.recipe.build
 slapos_promise =