Remove unnecessary parentheses
Mathematica, 105 97 91 bytes

-6 bytes thanks to Roman!

a=StringReplace;ToString@ToExpression@a[#,{"*"-&gt;"**","+"-&gt;"~~"}]~a~{" ** "-&gt;"*","~~"-&gt;"+"}&amp;


Replaces + and * with ~~ (StringExpression) and ** (NonCommutativeMultiply) respectively, evaluates it, stringifies it, and replaces the operators back.