Remove overloads of NonNullUtils#checkNotNull()

Remove overloads of NonNullUtils#checkNotNull()

New errors have cropped up with the latest integration build. It seems
JDT is now stricter with regards to annotated generic types vs.
un-annotated ones (where before un-annotated was similar to @Nullable,
now we don't we away with it that easily).

Variants of checkNotNull() method that used to help the compiler now
seem to do more harm than good. Just deferring to the basic "T" one
fixes the new errors. Perhaps this should have been the ideal approach
all along?

Change-Id: If998d5554b5d85ac785d21365dfeeae5d6a48bb8
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/60650
Reviewed-by: Hudson CI