git-gui: remove HEAD detachment implementation for git < 1.5.3

git-gui provides an implementation to detach HEAD on Git versions prior
to 1.5.3.  Nobody should be using such an old version anymore.
(Moreover, since 0730a5a, git-gui requires git v2.36 or later).
Keep only the code for modern Git.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
[j6t: message tweaked]
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Signed-off-by: Taylor Blau <me@ttaylorr.com>

Author: mark987

lib/checkout_op.tcl

@@ -510,18 +510,8 @@ method _update_repo_state {} {
 	delete_this
 }
 
-git-version proc _detach_HEAD {log new} {
-	>= 1.5.3 {
-		git update-ref --no-deref -m $log HEAD $new
-	}
-	default {
-		set p [gitdir HEAD]
-		file delete $p
-		set fd [open $p w]
-		fconfigure $fd -translation lf -encoding utf-8
-		puts $fd $new
-		close $fd
-	}
+proc _detach_HEAD {log new} {
+	git update-ref --no-deref -m $log HEAD $new
 }
 
 method _confirm_reset {cur} {