Fix math.map/math.lerp merge fallout (#1621)

- LuauMathMap flag was not removed but the uses of it were
- LuauMathLerp addition to math table was duplicated

VM/src/lmathlib.cpp

@@ -7,7 +7,6 @@
 #include <math.h>
 #include <time.h>
 
-LUAU_FASTFLAGVARIABLE(LuauMathMap)
 LUAU_FASTFLAGVARIABLE(LuauMathLerp)
 
 #undef PI
@@ -490,11 +489,5 @@ int luaopen_math(lua_State* L)
         lua_setfield(L, -2, "lerp");
     }
 
-    if (FFlag::LuauMathLerp)
-    {
-        lua_pushcfunction(L, math_lerp, "lerp");
-        lua_setfield(L, -2, "lerp");
-    }
-
     return 1;
 }