build: drop ldflags prereq goreleaser ignores

build-cross now runs goreleaser exclusively, and goreleaser sources
linker flags from .goreleaser.yaml (builds.ldflags), not the Makefile
LDFLAGS variable. The build-cross: LDFLAGS += -extldflags "-static"
prerequisite was a leftover from the mitchellh/gox era (075c096af)
and has had no effect since. Static linking is already set in
.goreleaser.yaml, so this only removes dead wiring.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

Makefile

@@ -170,7 +170,6 @@ $(GOIMPORTS):
 #  release
 
 .PHONY: build-cross
-build-cross: LDFLAGS += -extldflags "-static"
 build-cross: $(GORELEASER)
 	$(GORELEASER) build --snapshot --clean