lang/ruby30: Fix orphaned .timestamp in 93e3548c

When both CAPIDOCS and DOCS options are enabled, the following commands are executed:
1. post-install-CAPIDOCS-on: removed the installed .timestamp (in STAGEDIR).
2. post-install-DOCS-on: copied the generated .timestamp (in capi directory) to the STAGEDIR again.
Therefore, the installed .timestamp is orphaned.
To fix this, we remove both the generated and installed .timestamp in post-install-CAPIDOCS-on:.

PR:		267775
Reported by:	yasu