[hotfix] actually reduce socket pool size to 1

context:

• a prior MR (!334 (merged)) attempted to alleviate concurrency pressure on signald by reducing our socket pool size from 50 to 1
• however, it mistakenly changed vestigal configs in config.signal.poolSize, which is no longer used anywhere, thus having no effect at all on the pool size

changes:

• change the pool size in config.socket.poolSize (which is actually used)
• remove the vestigal signal config (which is not used anywhere)