Remove old Go fetch-pokeapi tool, update README for import-pokedb

The Go tool in tools/fetch-pokeapi/ has been fully replaced by the
Python import-pokedb tool. Removes all Go source files, updates the
README seed regeneration docs, and cleans up the .gitignore entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -64,8 +64,6 @@ temp/
 .pokeapi_cache/
 .pokedb_cache/
 
-# Go build output
-tools/fetch-pokeapi/fetch-pokeapi
 
 # Local config overrides
 *.local