chore: remove obsolete splinter SQL files and build script

The combined SQL files (splinter.sql, splinter_generic.sql, splinter_supabase.sql)
are no longer used by the runtime. The new system reads individual SQL files
from vendor/performance/ and vendor/security/ directories.

Changes:
- Remove obsolete combined SQL files
- Remove build.rs that generated the combined files
- Remove ureq build dependency

The individual rule SQL files in vendor/ subdirectories are maintained manually
and embedded at compile time via include_str! in the registry.

Author: psteinroe

Cargo.lock

@@ -18,9 +18,6 @@ serde.workspace             = true
 serde_json.workspace        = true
 sqlx.workspace              = true
 
-[build-dependencies]
-ureq = "2.10"
-
 [dev-dependencies]
 insta.workspace           = true
 pgls_console.workspace    = true