Revert "[libcxx] Fix xsgetn in basic_filebuf (#167779)"

This reverts commit ea16f7d.

Author: rupprecht

libcxx/include/fstream

@@ -315,14 +315,8 @@ protected:
         traits_type::copy(__str, this->gptr(), __n);
         this->__gbump_ptrdiff(__n);
       }
-      const streamsize __remainder    = __len - __n;
-      const streamsize __buffer_space = this->egptr() - this->eback();
-
-      if (__remainder >= __buffer_space)
-        return std::fread(__str + __n, sizeof(char_type), __remainder, __file_) + __n;
-      else if (__remainder > 0)
-        return basic_streambuf<_CharT, _Traits>::xsgetn(__str + __n, __remainder) + __n;
-      return __n;
+      if (__len - __n >= this->egptr() - this->eback())
+        return std::fread(__str + __n, sizeof(char_type), __len - __n, __file_);
     }
     return basic_streambuf<_CharT, _Traits>::xsgetn(__str, __len);
   }