Remove `spring.spel.ignore` and `spring.xml.ignore` flags

[sdeleuze]

Those undocumented flags were introduced for Spring Native experimental project needs but:

• With Java 17 baseline, XML mashalling/unmarshalling support now requires an additional dependency like org.glassfish.jaxb:jaxb-runtime so the regular classpath check should now work as expected.
• Removing SpEL support from the runtime path has a limited impact, and the right why to provide an efficient optimization should be to perform SpEL evaluation via Ahead Of Time transformation (not yet supported).

As a consequence, we are removing those flags from Spring Framework 6.