DeleteCallbackManager() introduced for better symmetry in the caller, they
do the same thing.

==20085== 24 bytes in 1 blocks are still reachable in loss record 11 of 103
==20085==    at 0x4C2A4CD: malloc (vg_replace_malloc.c:236)
==20085==    by 0x43A097: CreateCallbackList (dixutils.c:837)
==20085==    by 0x43A1D3: AddCallback (dixutils.c:869)
==20085==    by 0x4B1736: GEExtensionInit (geext.c:209)
==20085==    by 0x41C8A8: InitExtensions (miinitext.c:389)
==20085==    by 0x5AC918: main (main.c:208)

==2042== 8 bytes in 1 blocks are still reachable in loss record 2 of 97
==2042==    at 0x4C2A4CD: malloc (vg_replace_malloc.c:236)
==2042==    by 0x4C2A657: realloc (vg_replace_malloc.c:525)
==2042==    by 0x4802F5: XNFrealloc (utils.c:1095)
==2042==    by 0x43A17A: CreateCallbackList (dixutils.c:855)
==2042==    by 0x43A1EF: AddCallback (dixutils.c:870)
==2042==    by 0x4B1752: GEExtensionInit (geext.c:209)
==2042==    by 0x41C8A8: InitExtensions (miinitext.c:389)
==2042==    by 0x5AC9E4: main (main.c:208)
==2042==

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>