Fix Unix preallocatedArgCount

This has been an existing ToDo for some time.
This results in 19 bytes reduction in code size for pmi diffs of tests and frameworks.

Author: CarolEidt

src/jit/morph.cpp

@@ -4351,17 +4351,6 @@ GenTreeCall* Compiler::fgMorphArgs(GenTreeCall* call)
 
 #if defined(UNIX_AMD64_ABI)
         opts.compNeedToAlignFrame = true; // this is currently required for the UNIX ABI to work correctly
-
-        // ToDo: Remove this re-calculation preallocatedArgCount and use the value assigned above.
-
-        // First slots go in registers only, no stack needed.
-        // TODO-Amd64-Unix-CQ This calculation is only accurate for integer arguments,
-        // and ignores floating point args (it is overly conservative in that case).
-        preallocatedArgCount = nonRegPassedStructSlots;
-        if (argSlots > MAX_REG_ARG)
-        {
-            preallocatedArgCount += argSlots - MAX_REG_ARG;
-        }
 #endif // UNIX_AMD64_ABI
 
         const unsigned outgoingArgSpaceSize = preallocatedArgCount * REGSIZE_BYTES;