cmLocalGenerator: Remove unused MaybeRelativeToCurSrcDir method

With the recent update to `GetObjectFileNameWithoutTarget`, we no longer
have any call sites for `MaybeRelativeToCurSrcDir`.  It does not make
sense for the generator to produce paths relative to the source tree in
general, so remove the method.

Source/cmLocalGenerator.cxx

@@ -3698,12 +3698,6 @@ std::string cmLocalGenerator::MaybeRelativeToCurBinDir(
   return this->MaybeRelativeTo(this->GetCurrentBinaryDirectory(), path);
 }
 
-std::string cmLocalGenerator::MaybeRelativeToCurSrcDir(
-  std::string const& path) const
-{
-  return this->MaybeRelativeTo(this->GetCurrentSourceDirectory(), path);
-}
-
 std::string cmLocalGenerator::GetTargetDirectory(
   const cmGeneratorTarget* /*unused*/) const
 {

Source/cmLocalGenerator.h

@@ -471,7 +471,6 @@ public:
    */
   std::string MaybeRelativeToTopBinDir(std::string const& path) const;
   std::string MaybeRelativeToCurBinDir(std::string const& path) const;
-  std::string MaybeRelativeToCurSrcDir(std::string const& path) const;
 
   /**
    * Generate a macOS application bundle Info.plist file.