app-text/asciidoc: remove vim-syntax from IUSE

app-text/asciidoc: remove vim-syntax from IUSE

The vim-syntax USE flag is useless now, for the following reasons:

- The actions it controls have no effect, since the Makefile.in doesn't do
  anything vim-related anymore, as mentioned in the CHANGELOG.
- Vim comes with its own syntax file since version 7.4, which is maintained (by
  AsciiDoc upstream, according to its header) and was last updated in 2014,
  whereas the upstream syntax file has not been changed since 2013.

Gentoo-bug: 530808
Signed-off-by: Marc Joliet <marcec@gmx.de>