Previous CLs (CL/54970, CL55231, and CL/55237) re-implemented tar.Writer
entirely using specialized methods (writeUSTARHeader, writePAXHeader,
and writeGNUHeader) allowing tar.Writer to entirely side-step the broken
and buggy logic in writeHeader.

Since writeHeader and writePAXHeaderLegacy is now dead-code,
we can delete them.

One minor change is that we call Writer.Flush at the start of WriteHeader.
This used to be performed by writeHeader, but doing so in WriteHeader
ensures each of the specialized methods can benefit from its effect.

Fixes #17665
Fixes #12594

Change-Id: Iff2ef8e7310d40ac5484d2f8852fc5df25201426
Reviewed-on: https://go-review.googlesource.com/55550Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>