Revert "Add ink-tab-width"

This reverts commit f2cd3f4.
Since tab-width is used for both spaces and tabs in ink-mode, using a
derived variable messes up the indentation calculations when using
tabs.

Bump version as well

Version 0.3.1

Version

Replace flycheck with flymake

Header: use patch version number, remove Package-Version

Merge pull request #3 from syohex/defgroup

Define group for customize and byte-compile warning