Remove preprocessor guards from injection points

Remove preprocessor guards from injection points

When defining an injection point there is no need to wrap the definition
with USE_INJECTION_POINT guards, the INJECTION_POINT macro is available
in all builds.  Remove to make the code consistent.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/OSCPR01MB14966C8015DEB05ABEF2CE077F51FA@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 17