Fix unshift_* templates (no destructive '!')

Remove destructive '!' character. As bdw explained in his feedback, the
'!' identifies an op that doesn't only fail to produce a value, but also
has a write register that is explicitly populated within the template.

Since the unshift ops don't write to a register, it is not correct to
mark it with '!'.

src/jit/core_templates.expr

@@ -987,7 +987,7 @@
       (carg $0 ptr)
       (carg (const (&QUOTE MVM_reg_obj) int_sz) int))))
 
-(template: unshift_i!
+(template: unshift_i
   (callv (^getf (^repr $0) MVMREPROps pos_funcs.unshift)
     (arglist
       (carg (tc) ptr)
@@ -997,7 +997,7 @@
       (carg $1 ptr)
       (carg (const (&QUOTE MVM_reg_int64) int_sz) int))))
 
-(template: unshift_n!
+(template: unshift_n
   (callv (^getf (^repr $0) MVMREPROps pos_funcs.unshift)
     (arglist
       (carg (tc) ptr)
@@ -1007,7 +1007,7 @@
       (carg $1 ptr)
       (carg (const (&QUOTE MVM_reg_num64) int_sz) int))))
 
-(template: unshift_s!
+(template: unshift_s
   (callv (^getf (^repr $0) MVMREPROps pos_funcs.unshift)
     (arglist
       (carg (tc) ptr)
@@ -1017,7 +1017,7 @@
       (carg $1 ptr)
       (carg (const (&QUOTE MVM_reg_str) int_sz) int))))
 
-(template: unshift_o!
+(template: unshift_o
   (callv (^getf (^repr $0) MVMREPROps pos_funcs.unshift)
     (arglist
       (carg (tc) ptr)