rs6000: Remove entry for V1TImode from VI_unit

When making a patch to adjust VECTOR_P8_VECTOR rs6000_vector
enum, I noticed that V1TImode's mode attribute in VI_unit
VECTOR_UNIT_ALTIVEC_P (V1TImode) is never true, since
VECTOR_UNIT_ALTIVEC_P checks if vector_unit[V1TImode] is
equal to VECTOR_ALTIVEC, but vector_unit[V1TImode] can only
be VECTOR_NONE or VECTOR_P8_VECTOR, there is no chance to be
VECTOR_ALTIVEC:
  rs6000_vector_unit[V1TImode]
      = (TARGET_P8_VECTOR) ? VECTOR_P8_VECTOR : VECTOR_NONE;

By checking all uses of VI_unit, the used mode iterator is
one of VI2, VI, VP_small and VP, none of them has V1TImode,
so the entry for V1TImode is useless.  I guessed it was
designed to have one mode attribute to cover all integer
vector modes, but later we separated V1TI handlings to its
own patterns (those guarded with TARGET_VADDUQM).  Anyway,
this patch is to remove this useless and confusing entry.

gcc/ChangeLog:

	* config/rs6000/altivec.md (mode attr for V1TI in VI_unit): Remove.