Document more TODOs as tickets (#1418)

Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for local tickets, `FIXME: DESC LINK` for external tickets) for easier in-editor support. Further more remove unnecessary remarks and related old code that I noticed in that instance.

substrate/core/sr-primitives/src/generic/unchecked_extrinsic.rs

@@ -156,7 +156,6 @@ impl<Address: Codec, Index: HasCompact + Codec, Signature: Codec, Call: Encode>
 	}
 }
 
-/// TODO: use derive when possible.
 #[cfg(feature = "std")]
 impl<Address, Index, Signature, Call> fmt::Debug
 	for UncheckedExtrinsic<Address, Index, Call, Signature>

substrate/core/sr-primitives/src/generic/unchecked_mortal_compact_extrinsic.rs

@@ -168,7 +168,6 @@ impl<Address: Encode, Index, Signature: Encode, Call: Encode> serde::Serialize
 	}
 }
 
-/// TODO: use derive when possible.
 #[cfg(feature = "std")]
 impl<Address, Index, Call, Signature> fmt::Debug for UncheckedMortalCompactExtrinsic<Address, Index, Call, Signature> where
 	Address: fmt::Debug,

substrate/core/sr-primitives/src/generic/unchecked_mortal_extrinsic.rs

@@ -166,7 +166,6 @@ impl<Address: Encode, Index: Encode, Signature: Encode, Call: Encode> serde::Ser
 	}
 }
 
-/// TODO: use derive when possible.
 #[cfg(feature = "std")]
 impl<Address, Index, Call, Signature> fmt::Debug for UncheckedMortalExtrinsic<Address, Index, Call, Signature> where
 	Address: fmt::Debug,