Revert "debian/rules: Set DPKG_GENSYMBOLS_CHECK_LEVEL = 4"

This reverts commit f4cd2d4d.

The extra checking is unnecessary since the default level 1
still catches any removed symbols.

The C++ symbols vary between architectures which adds extra work
and extra uploads if we don't get it quite right. The current
Debian maintainers aren't interested in doing all that extra work here.

Also, poppler bumps their soname so frequently that there isn't
a risk of breaking changes not being seen.

debian/rules

@@ -11,8 +11,6 @@ libpopplerN := $(shell sed -rn 's/^Package:[[:space:]]*(libpoppler[0-9]+)[[:spac
 # enable all the hardening options
 export DEB_BUILD_MAINT_OPTIONS = hardening=+all
 
-export DPKG_GENSYMBOLS_CHECK_LEVEL = 4
-
 # disable gtk stuff to minimize Build-Depends
 CONFIGURE_ARGS :=				\
 	-DCMAKE_BUILD_TYPE=Release		\