internal/sdref: Remove in favor of stdplus/handle

We no longer need to roll our own managed handle type now that stdplus
implements a generic one.

Tested:
    Built and run through unit tests.

Change-Id: Id34cd8d3ffacf7901d49cac335fa93f744f0e310
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/configure.ac b/configure.ac
index c725bcf..a933bb2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -39,6 +39,9 @@
 AX_APPEND_COMPILE_FLAGS([-Wall -Wextra -Wpedantic], [CFLAGS])
 AX_APPEND_COMPILE_FLAGS([-Wall -Wextra -Wpedantic], [CXXFLAGS])
 
+# We require stdplus unconditionally
+AX_PKG_CHECK_MODULES([STDPLUS], [stdplus])
+
 # We require systemd unconditionally
 AX_PKG_CHECK_MODULES([SYSTEMD], [libsystemd >= 218])