text/template/parse: rename DeferFuncCheck to SkipFuncCheck

The proposal as accepted in #34652 named the bit SkipFuncCheck.
It was renamed to DeferFuncCheck during the code review on a suggestion by Rob,
along with a comment to “defer type checking functions until template is executed,”
but this description is not accurate: the package has never type-checked functions,
only verified their existence. And the effect of the bit in this package is to eliminate
this check entirely, not to defer it to some later time.

I was writing code using this new bit and was very confused about when the
"type checking" was being deferred to and how to stop that entirely,
since in my use case I wanted no checks at all. What I wanted is what the bit does,
it just wasn't named accurately.

Rename back to SkipFuncCheck.

Change-Id: I8e62099c8a904ed04521eb5b86155290f6d5b12f
Reviewed-on: https://go-review.googlesource.com/c/go/+/317269


Trust: Russ Cox <rsc@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>