Fix #11 — `RCA_STAGE` macro no longer exists

In Battle for Wesnoth commit c4c0c553ec0b10ca2e5efaebb438b2fb7801feef,
mattsc deleted the obsolete `RCA_STAGE` macro from mainline.

This macro was used by NX in E1S3, so its deletion from mainline
resulted in undefined macro errors when preprocessing NX.

This commit resolves the issue by deleting E1S3 side 2’s `[ai]` tag,
including the `{RCA_STAGE}` macro inclusion, per mattsc’s instructions
(see #wesnoth-dev logs, 2014-01-03 23:51 UTC).