Enable LTO again

It had reportedly been disabled due to failing CppunitTest_sw_apitests in
LO 6.4.  My recent upstream check with --enable-lto on master towards LO 7.1
found CppunitTest_sw_apitests to work fine, and only found one other failing
test in CppunitTest,xmlsecurity_signing, but which is new for LO 7.1 and not yet
present in LO 7.0 (see <https://git.libreoffice.org/core/+/
800eebfa82106c509310ed43bef38a7a4ad4451f%5E!> "Database document apparently
needs to be closed before it is disposed" and <https://git.libreoffice.org/core/
+/2f2246d22e2a8ccbc1dc3e6f5243734a61edf270%5E!> "external/cppunit: Run tests in
deterministic order".

From a successful scratch build of this commit, it looks like
CppunitTest_sw_apitests already works fine in LO 7.0, and that there are no
other issue with LTO in LO 7.0.

libreoffice.spec

@@ -999,10 +999,6 @@ mv .git .git-rpm
 # path to external tarballs
 EXTSRCDIR=`dirname %{SOURCE0}`
 
-# This package fails its testsuite when LTO is enabled.
-# see: https://bugs.documentfoundation.org/show_bug.cgi?id=126442
-%define _lto_cflags %{nil}
-
 #use the RPM_OPT_FLAGS but remove the LibreOffice overridden ones
 for i in $RPM_OPT_FLAGS; do
         case "$i" in