Obsolete comment in sv.c:utf8_mg_pos_cache_update

ab8be49d3 added this comment and the SvPOKp check.  Half the comment
is not relevant any more, since we stopped caching UTF8 offsets on
references in commit d91e94e1ac8.  The SvPOKp check is still a good
idea, though.

$ perl5.14.4 -e 'use overload q|""| => sub { "\x{100}" }; $_ = bless[]; $__ = substr $_,0,1; use Devel::Peek; Dump $_'
SV = PVMG(0x7fb2f2856738) at 0x7fb2f283a348
  REFCNT = 1
  FLAGS = (SMG,ROK,UTF8)
  IV = 0
  NV = 0
  RV = 0x7fb2f2806d00
  SV = PVAV(0x7fb2f2807ec0) at 0x7fb2f2806d00
    REFCNT = 1
    FLAGS = (OBJECT,OVERLOAD)
    STASH = 0x7fb2f2806b68	"main"
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)
  PV = 0x7fb2f2806d00 "" [UTF8 ""]
  CUR = 0
  LEN = 0
  MAGIC = 0x7fb2f240f708
    MG_VIRTUAL = &PL_vtbl_utf8
    MG_TYPE = PERL_MAGIC_utf8(w)
    MG_LEN = 1
$ perl5.18.1 -e 'use overload q|""| => sub { "\x{100}" }; $_ = bless[]; $__ = substr $_,0,1; use Devel::Peek; Dump $_'
SV = IV(0x7fc048837c40) at 0x7fc048837c50
  REFCNT = 1
  FLAGS = (ROK,UTF8)
  RV = 0x7fc0488060e8
  SV = PVAV(0x7fc048807e80) at 0x7fc0488060e8
    REFCNT = 1
    FLAGS = (OBJECT)
    STASH = 0x7fc048805f50	"main"
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)