Remove links to obsolete/removed techniques

[kfranqueiro]

The following links to removed Flash techniques currently lead to 404s, as they are output by the XSLT build process even though they cannot be resolved (as evidenced by the lack of title in their links):

understanding/20/link-purpose-in-context.html: skipping unresolvable technique id FLASH7
understanding/20/link-purpose-in-context.html: skipping unresolvable technique id FLASH5
understanding/20/link-purpose-link-only.html: skipping unresolvable technique id FLASH7
understanding/20/link-purpose-link-only.html: skipping unresolvable technique id FLASH5

Additionally, the following link to an obsolete technique also does not resolve during the build process, but technically hasn't been deleted from the public site, so it still works:

techniques/general/G192.html: skipping unresolvable technique id H75

The Eleventy build process in #3917 will avoid outputting these links (and generates the warning messages seen here), but they should still be cleaned up in the source files.

[patrickhlauke]

Will be addressed by #3540