Don't check for temporary values on non-key properties when deleting an entity

[AndriySvyryd]

See npgsql/efcore.pg#518 (comment)

[NinoFloris]

@ajcvickers why did this get pushed into backlog? Postgresql seeding with xmin still doesn't work after 2 years of seeding being introduced. There is a tail of issues slowly fixing parts of the problem, this being the last piece. I'd really appreciate it if 3.0 or a 3.0 patch would finally fix this.

[ajcvickers]

@AndriySvyryd @roji Thoughts?

[roji]

@NinoFloris this issue isn't actually necessary in order to seed entities that has xmin. The root issue was #14161 which was fixed in EF Core 2.2.2 - this issue was opened as part of a proposed workaround for that, which is no longer necessary to use xmin (but is important for other scenarios).