lib/model: Use a single lock (phase two: cleanup) (#9276)

Cleanup after #9275.

This renames `fmut` -> `mut`, removes the deadlock detector and
associated plumbing, renames some things from `...PRLocked` to
`...RLocked` and similar, and updates comments.

Apart from the removal of the deadlock detection machinery, no
functional code changes... i.e. almost 100% diff noise, have fun
reviewing.

lib/sync/sync.go

@@ -15,8 +15,6 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
-
-	"github.com/sasha-s/go-deadlock"
 )
 
 var timeNow = time.Now
@@ -39,9 +37,6 @@ type WaitGroup interface {
 }
 
 func NewMutex() Mutex {
-	if useDeadlock {
-		return &deadlock.Mutex{}
-	}
 	if debug {
 		mutex := &loggedMutex{}
 		mutex.holder.Store(holder{})
@@ -51,9 +46,6 @@ func NewMutex() Mutex {
 }
 
 func NewRWMutex() RWMutex {
-	if useDeadlock {
-		return &deadlock.RWMutex{}
-	}
 	if debug {
 		mutex := &loggedRWMutex{
 			readHolders: make(map[int][]holder),