[NFC][webkit.UncountedLambdaCapturesChecker] Remove unnecessary check (#120069)

CXXMD is checked for null, but it can't be null inside of a visitor's
method. Found by a static analyzer tool.

Author: Fznamznon

clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedLambdaCapturesChecker.cpp

@@ -51,7 +51,7 @@ class UncountedLambdaCapturesChecker
 
       bool TraverseCXXMethodDecl(CXXMethodDecl *CXXMD) override {
         llvm::SaveAndRestore SavedDecl(ClsType);
-        if (CXXMD && CXXMD->isInstance())
+        if (CXXMD->isInstance())
           ClsType = CXXMD->getThisType();
         return DynamicRecursiveASTVisitor::TraverseCXXMethodDecl(CXXMD);
       }