git-gui: remove unused proc is_shellscript

Commit 7d076d5675 (git-gui: handle shell script text filters when
loading for blame, 2011-12-09) added is_shellscript to test if a file
is executable by the shell, used only when searching for textconv
filters. The previous commit rearranged the tests for finding such
filters, and removed the only user of is_shellscript. Remove this
function.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

git-gui.sh

@@ -549,16 +549,6 @@ proc _git_cmd {name} {
 	return $v
 }
 
-# Test a file for a hashbang to identify executable scripts on Windows.
-proc is_shellscript {filename} {
-	if {![file exists $filename]} {return 0}
-	set f [open $filename r]
-	fconfigure $f -encoding binary
-	set magic [read $f 2]
-	close $f
-	return [expr {$magic eq "#!"}]
-}
-
 # Run a shell command connected via pipes on stdout.
 # This is for use with textconv filters and uses sh -c "..." to allow it to
 # contain a command with arguments. We presume this