Remove specialization hacks for `PopArgument` and `SlashArgument` (#357) #1272

Job	Run time
Build docs	2m 12s
Test (MSRV)	1m 21s
Test (beta)	1m 44s
Test (nightly)	1m 38s
Test (stable)	1m 46s
Test (Windows)	4m 8s
Test (no features)	1m 6s
Clippy	49s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 11s
Test (macOS)	1m 57s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 18s
Rustfmt	4s
	19m 14s

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