build-sys: Remove --enable-experimental-api

It was added for the collections bits, but we made that stable.
It's now just cruft and we're very unlikely to reuse the infrastructure
again.

Motivated by a unit test failure when running from a tarball:
https://github.com/ostreedev/ostree/issues/2313

.cci.jenkinsfile

@@ -48,7 +48,7 @@ minimal: {
         ./configure --without-curl --without-soup --disable-gtk-doc --disable-man \
           --disable-rust --without-libarchive --without-selinux --without-smack \
           --without-openssl --without-avahi --without-libmount --disable-rofiles-fuse \
-          --disable-experimental-api --without-libsodium
+          --without-libsodium
         make
       """)
   }