Remove warning about extraneous "ruby-" prefix in `.ruby-version`

When we started to support reading `.ruby-version` files, we made a
commitment to not support fuzzy version matching. Treating "ruby-2.1.5"
as "2.1.5" is a sort of fuzzy matching, so we put in place a warning
telling you to remove the extraneous "ruby-" prefix popularly used by
other Ruby version managers to denote MRI. (Their logic is that MRI is
"ruby" and other rubies are not "ruby", apparently.)

However, people are often not able to remove the prefix in their
projects because they want to support other coworkers and tools that
sadly still require the prefix, like RubyMine.

So to restore sanity for a big portion of our users, the warning is gone.

libexec/rbenv-version-name

@@ -21,9 +21,6 @@ version_exists() {
 if version_exists "$RBENV_VERSION"; then
   echo "$RBENV_VERSION"
 elif version_exists "${RBENV_VERSION#ruby-}"; then
-  { echo "warning: ignoring extraneous \`ruby-' prefix in version \`${RBENV_VERSION}'"
-    echo "         (set by $(rbenv-version-origin))"
-  } >&2
   echo "${RBENV_VERSION#ruby-}"
 else
   echo "rbenv: version \`$RBENV_VERSION' is not installed" >&2

test/version-name.bats

@@ -57,9 +57,5 @@ setup() {
   cat > ".ruby-version" <<<"ruby-1.8.7"
   run rbenv-version-name
   assert_success
-  assert_output <<OUT
-warning: ignoring extraneous \`ruby-' prefix in version \`ruby-1.8.7'
-         (set by ${PWD}/.ruby-version)
-1.8.7
-OUT
+  assert_output "1.8.7"
 }