Takes advantage of CL 83740044, to optimize map[string] lookup
from []byte key.

Deletes code.

No conditional check for gccgo, since Ian plans to add this
to gccgo before GCC 4.10 (Go 1.3).

benchmark                   old ns/op     new ns/op     delta
BenchmarkReadMIMEHeader     6066          5086          -16.16%

benchmark                   old allocs     new allocs     delta
BenchmarkReadMIMEHeader     12             12             +0.00%

benchmark                   old bytes     new bytes     delta
BenchmarkReadMIMEHeader     1317          1317          +0.00%

Update #3512

LGTM=rsc
R=rsc, dave
CC=golang-codereviews, iant
https://golang.org/cl/84230043