Revert "Add new @ shortcut for HEAD"

This reverts commit cdfd94837b, as it
does not just apply to "@" (and forms with modifiers like @{u}
applied to it), but also affects e.g. "refs/heads/@/foo", which it
shouldn't.

The basic idea of giving a short-hand might be good, and the topic
can be retried later, but let's revert to avoid affecting existing
use cases for now for the upcoming release.

Documentation/git-check-ref-format.txt

@@ -54,8 +54,6 @@ Git imposes the following rules on how references are named:
 
 . They cannot contain a sequence `@{`.
 
-. They cannot be the single character `@`.
-
 . They cannot contain a `\`.
 
 These rules make it easy for shell script based tools to parse

Documentation/revisions.txt

@@ -58,9 +58,6 @@ the '$GIT_DIR/refs' directory or from the '$GIT_DIR/packed-refs' file.
 While the ref name encoding is unspecified, UTF-8 is preferred as
 some output processing may assume ref names in UTF-8.
 
-'@'::
-  '@' alone is a shortcut for 'HEAD'.
-
 '<refname>@\{<date>\}', e.g. 'master@\{yesterday\}', 'HEAD@\{5 minutes ago\}'::
   A ref followed by the suffix '@' with a date specification
   enclosed in a brace