sane-backends.pc.in: Drop ldflags from the file

The ldflags variable is no longer used in the file because it embedded
LDFLAGS from build environment and LibGPhoto2 LDFLAGS, where the former
contains flags from the environment and can cause problems, and the
latter is empty.

With the state above, we can drop ldflags from pkgconfig file
completely.

tools/sane-backends.pc.in

@@ -2,7 +2,6 @@ prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
-ldflags=@LDFLAGS@ @GPHOTO2_LDFLAGS@
 libs=@LIBS@ @DL_LIBS@ @LIBV4L_LIBS@ @MATH_LIB@ @TIFF_LIBS@ @JPEG_LIBS@ @GPHOTO2_LIBS@ @SOCKET_LIBS@ @AVAHI_LIBS@ @USB_LIBS@ @SCSI_LIBS@ @RESMGR_LIBS@
 
 Name: SANE Backends
@@ -10,5 +9,5 @@ Description: Backends for SANE, the universal scanner interface
 Version: @VERSION@
 Requires:
 Libs: -L${libdir} -lsane
-Libs.private: ${ldflags} ${libs}
+Libs.private: ${libs}
 Cflags: -I${includedir}