Blender Git Loki

Cleanup: remove KM_MOD_SECOND & KM_SHIFT2, KM_CTRL2.. etc

These were added in a1c8543f2acd7086d412cb794b32f96794b00659 (2007)
but never used.
Nor did they have any meaning in practice.

Note that versioning keymap items isn't needed as these values were
never set. The code-paths that set these values also set KM_MOD_FIRST
causing `keymap_event_set` to only ever assign values of 0 or 1.

These flags complicate further exposing KM_ANY (-1)
which is also a valid value for modifiers.