Re #6543. Remove unnecessary guard that fails on the Mac.

And the test that checks it. If the file doesn't exist, it
won't be possible to execute the algorithm as the validator on
the Filename property will prevent it.

Code/Mantid/Framework/DataHandling/src/LoadDetectorInfo.cpp

@@ -805,10 +805,6 @@ void LoadDetectorInfo::sometimesLogSuccess(const detectorInfo &params, bool &nee
 /**The methor reads selected part of detector.nxs file and apply correspondent changes to the detectors */ 
 void LoadDetectorInfo::readNXS(const std::string& fName)
 {
-  Poco::File nxsFile(fName);
-  if(!nxsFile.exists() || !nxsFile.isFile())
-    throw std::invalid_argument(" file "+fName+" does not exist");
-
   auto hFile = new ::NeXus::File(fName,NXACC_READ);
   if(!hFile)
     throw std::runtime_error(" Can not open file "+fName+" as nexus file");

Code/Mantid/Framework/DataHandling/test/LoadDetectorInfoTest.h

@@ -275,8 +275,6 @@ class LoadDetectorInfoTest : public CxxTest::TestSuite
   {
     LoadDetectorIndoTestHelper loader;
 
-    TS_ASSERT_THROWS(loader.readNXS("NonExistingFile"),std::invalid_argument);
-
     std::string nexusWithoutDet = FileFinder::Instance().getFullPath("argus0026287.nxs");
     if(nexusWithoutDet.empty())return;
     TSM_ASSERT_THROWS("this nexus does not have detector information ",loader.readNXS(nexusWithoutDet),std::invalid_argument);