Disassembler: Remove duplicated "signature" in output (microsoft#6726)

There is an extra "signature" in the Patch Constant signature part of
the disassembler output, causing the "Patch Constant signature
signature" to appear. This PR removes the extra "signature" from the
output.

Example HLSL input:

```hlsl
struct OutputConstantData {
    float tessFactor[4] : SV_TessFactor;
    float insideTessFactor[2] : SV_InsideTessFactor;
};

OutputConstantData HSConstant() {
    OutputConstantData output;
    return output;
}

[domain("quad")]
[partitioning("integer")]
[outputtopology("triangle_cw")]
[outputcontrolpoints(1)]
[patchconstantfunc("HSConstant")]
void HSMain() {}
```

Example Disassembler output:

```diff
  ...
  ;
- ; Patch Constant signature signature:
+ ; Patch Constant signature:
  ;
  ; Name                 Index   Mask Register SysValue  Format   Used
  ; -------------------- ----- ------ -------- -------- ------- ------
  ; SV_TessFactor            0      w        0 QUADEDGE   float      w
  ; SV_TessFactor            1      w        1 QUADEDGE   float      w
  ; SV_TessFactor            2      w        2 QUADEDGE   float      w
  ; SV_TessFactor            3      w        3 QUADEDGE   float      w
  ; SV_InsideTessFactor      0      w        4  QUADINT   float      w
  ; SV_InsideTessFactor      1      w        5  QUADINT   float      w
  ;
  ...
```

Author: SuperSodaSea

tools/clang/tools/dxcompiler/dxcdisassembler.cpp

@@ -1681,7 +1681,7 @@ HRESULT Disassemble(IDxcBlob *pProgram, raw_string_ostream &Stream) {
                       DxilPartIsType(DFCC_PatchConstantSignature));
     if (it != end(pContainer)) {
       PrintSignature(
-          "Patch Constant signature",
+          "Patch Constant",
           reinterpret_cast<const DxilProgramSignature *>(GetDxilPartData(*it)),
           false, Stream, /*comment*/ ";");
     }