gdalallregister.cpp: Finish removing rda driver.

Follow on to these commits:

- 6799268 - Remove R Object Data Store (*.rda) driver
- a4de9c0 Merge pull request OSGeo#11758 from rouault/remove_geoconcept
  - This was cumulative with previous PRs and contains the removal of rda

frmts/gdalallregister.cpp

@@ -563,10 +563,6 @@ void CPL_STDCALL GDALAllRegister()
     GDALRegister_COASP();
 #endif
 
-#ifdef FRMT_r
-    GDALRegister_R();
-#endif
-
 #ifdef FRMT_map
     GDALRegister_MAP();
 #endif