Drop the clang patch which added a custom vendor suffix to the version

printed with -v.  We have historically put a date stamp there (roughly
corresponding to the date of import), but this has never been used for
anything, and the patch has also never been upstreamed, so let's get rid
of it now.

contrib/llvm/patches/patch-01-clang-vendor-suffix.diff

@@ -1,22 +0,0 @@
-This patch adds a FreeBSD-specific suffix to clang's version string.  This is
-usually of the form "(yyyyddmm)", representing the date when the compiler was
-last updated.
-
-Introduced here: http://svnweb.freebsd.org/changeset/base/209107
-
-Index: tools/clang/lib/Basic/Version.cpp
-===================================================================
---- tools/clang/lib/Basic/Version.cpp
-+++ tools/clang/lib/Basic/Version.cpp
-@@ -128,8 +128,10 @@ std::string getClangToolFullVersion(StringRef Tool
-   OS << ToolName << " version " CLANG_VERSION_STRING " "
-      << getClangFullRepositoryVersion();
- 
-+#ifdef CLANG_VENDOR_SUFFIX
-+  OS << CLANG_VENDOR_SUFFIX;
-+#elif defined(CLANG_VENDOR)
-   // If vendor supplied, include the base LLVM version as well.
--#ifdef CLANG_VENDOR
-   OS << " (based on " << BACKEND_PACKAGE_STRING << ")";
- #endif
- 

contrib/llvm/tools/clang/lib/Basic/Version.cpp

@@ -128,10 +128,8 @@ std::string getClangToolFullVersion(StringRef ToolName) {
   OS << ToolName << " version " CLANG_VERSION_STRING " "
      << getClangFullRepositoryVersion();
 
-#ifdef CLANG_VENDOR_SUFFIX
-  OS << CLANG_VENDOR_SUFFIX;
-#elif defined(CLANG_VENDOR)
   // If vendor supplied, include the base LLVM version as well.
+#ifdef CLANG_VENDOR
   OS << " (based on " << BACKEND_PACKAGE_STRING << ")";
 #endif