build: Remove Jansson dependency

After previous changes, this library is no longer used.  We have
switched to json-c, for better compatibility with libvirt.

(cherry picked from
guestfs-tools commit e6dcf7e3a7e9170978e57ce6df1b34f92fac5ae3)

docs/guestfs-building.pod

@@ -185,10 +185,6 @@ I<Required>.
 
 I<Required>.
 
-=item Jansson E<ge> 2.7
-
-I<Required>.
-
 =item json-c E<ge> 0.14
 
 I<Required>.

lib/Makefile.am

@@ -138,7 +138,6 @@ libguestfs_la_CFLAGS = \
 	$(PCRE2_CFLAGS) \
 	$(LIBVIRT_CFLAGS) \
 	$(LIBXML2_CFLAGS) \
-	$(JANSSON_CFLAGS) \
 	$(JSON_C_CFLAGS)
 
 libguestfs_la_LIBADD = \
@@ -150,7 +149,6 @@ libguestfs_la_LIBADD = \
 	$(PCRE2_LIBS) \
 	$(LIBVIRT_LIBS) $(LIBXML2_LIBS) \
 	$(SELINUX_LIBS) \
-	$(JANSSON_LIBS) \
 	$(JSON_C_LIBS) \
 	../gnulib/lib/libgnu.la \
 	$(LIBSOCKET) \

m4/guestfs-libraries.m4

@@ -301,9 +301,6 @@ LIBS="$LIBS $LIBXML2_LIBS"
 AC_CHECK_FUNCS([xmlBufferDetach])
 LIBS="$old_LIBS"
 
-dnl Check for Jansson JSON library (required).
-PKG_CHECK_MODULES([JANSSON], [jansson >= 2.7])
-
 dnl Check for JSON-C library (required).
 PKG_CHECK_MODULES([JSON_C], [json-c >= 0.14])