git-gui: Remove unused icon file_parttick

This icon hasn't been used in git gui.  I think it dates back to
the original set of icons I took from Paul Mackerras' prototype
that I turned into git gui.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1855,15 +1855,6 @@ static unsigned char file_fulltick_bits[] = {
    0x62, 0x10, 0x02, 0x10, 0xfe, 0x1f};
 } -maskdata $filemask
 
-image create bitmap file_parttick -background white -foreground "#005050" -data {
-#define parttick_width 14
-#define parttick_height 15
-static unsigned char parttick_bits[] = {
-   0xfe, 0x01, 0x02, 0x03, 0x7a, 0x05, 0x02, 0x09, 0x7a, 0x1f, 0x02, 0x10,
-   0x7a, 0x14, 0x02, 0x16, 0x02, 0x13, 0x8a, 0x11, 0xda, 0x10, 0x72, 0x10,
-   0x22, 0x10, 0x02, 0x10, 0xfe, 0x1f};
-} -maskdata $filemask
-
 image create bitmap file_question -background white -foreground black -data {
 #define file_question_width 14
 #define file_question_height 15