To remove a couple of unwanted wrapper methods

To check for assignment overflows.

Looks like static methods that do not get inlined end up exported.
We have a lot more.  Need to protect all at some point.  Wish there
was an easier way, like the visibility flag we pass that automatically
hides all inline methods.

Was exposed by check-symbols.sh when compiling on OS X 10.14 with:

$ make CPPFLAGS=-Oz CXXFLAGS=-flto=thin LDFLAGS=-lc++

Not that useful.

Fuzzer caught it:

==14==ERROR: AddressSanitizer: stack-use-after-return on address 0x7fca2ed7a3e0 at pc 0x0000006057aa bp 0x7ffc3290f1d0 sp 0x7ffc3290f1c8
READ of size 4 at 0x7fca2ed7a3e0 thread T0
SCARINESS: 55 (4-byte-read-stack-use-after-return)
    #0 0x6057a9 in OT::SingleSubstFormat2::subset(hb_subset_context_t*) const /src/harfbuzz/src/./hb-ot-layout-gsub-table.hh:194:40
    #1 0x5ff921 in hb_subset_context_t::return_t OT::SingleSubst::dispatch<hb_subset_context_t>(hb_subset_context_t*) const /src/harfbuzz/src/./hb-ot-layout-gsub-table.hh:256:13

I can't reproduce locally, but many of the bots are failing because of this
as well.

It's a pity that operator-> must return pointer.  Ugh.  Why?!

Part of https://github.com/harfbuzz/harfbuzz/issues/1553

Makes it nicer in pipelines.