fix(database): tag-only-change triggers update + add RemoveTag call

[wotolom]

Description of your changes

for database.RDSInstance:

• fix isUpToDate() logic to trigger update on tag-only-change (used cache scheme/solution from rds.DBInstance)
• add RemoveTagsFromResource() from AWS API to update() (removing tags was apparently not possible before...)
• adapt unit-tests

Fixes #1962

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

manually (update tag-only, update other db parameter)

[wotolom]

I do not really understand, why e.cache behaves here differently then err:

upToDate, diff, e.cache, err := rds.IsUpToDate(ctx, e.kube, cr, instance)

is not possible:

non-name e.cache on left side of :=compiler(BadDecl)

therefore I did this:

provider-aws/pkg/controller/database/rdsinstance/rdsinstance.go

Lines 158 to 162 in f7297fa

	var cachecache rds.Cache
	upToDate, diff, cachecache, err := rds.IsUpToDate(ctx, e.kube, cr, instance)
	e.cache = cachecache

[MisterMX]

I do not really understand, why e.cache behaves here differently then err:

:= redeclares a new variable which is not possible with e.cache. You have to use = and declare other new variables beforehand if you want to use something like e.cache = rds.IsUpToDate`.

[MisterMX]

LGTM. Thank you very much @wotolom!