Bug 1856795 - Remove redundant member init r=emilio

Done with:
./mach static-analysis check --checks="-*, readability-redundant-member-init" --fix .

https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-member-init.html

Differential Revision: https://phabricator.services.mozilla.com/D190002

modules/libpref/Preferences.cpp

@@ -2293,10 +2293,7 @@ class nsPrefLocalizedString final : public nsIPrefLocalizedString {
 //----------------------------------------------------------------------------
 
 nsPrefBranch::nsPrefBranch(const char* aPrefRoot, PrefValueKind aKind)
-    : mPrefRoot(aPrefRoot),
-      mKind(aKind),
-      mFreeingObserverList(false),
-      mObservers() {
+    : mPrefRoot(aPrefRoot), mKind(aKind), mFreeingObserverList(false) {
   nsCOMPtr<nsIObserverService> observerService = services::GetObserverService();
   if (observerService) {
     ++mRefCnt;  // must be > 0 when we call this, or we'll get deleted!