Remove build-context from make fixtures

[damemi]

This flag was added in #1108 and I am not sure if that was intentional or not. Running it as it is, I get the following error:

make[1]: Leaving directory '/usr/local/google/home/mikedame/go/src/github.com/open-telemetry/opentelemetry-go-instrumentation'                    
if [ -f ./internal/test/e2e/grpc/build.sh ]; then \
	./internal/test/e2e/grpc/build.sh; \
else \
	cd internal/test/e2e/grpc && docker build --build-context ../../../../ -t sample-app . ;\
fi
unknown flag: --build-context
See 'docker build --help'.
make: *** [Makefile:167: fixtures/grpc] Error 125

Changing it to docker buildx build showed that it's actually supposed to be a key=value pair, so it doesn't seem to be used anyway:

ERROR: invalid context value: ../../../../, expected key=value
make: *** [Makefile:167: fixtures/grpc] Error 1

I don't think this was caught because our e2es don't actually call make fixtures, instead they re-build essentially the same steps without the need for docker

[damemi]

cc @MrAlias please correct me if I'm wrong and this has a purpose I'm missing

[MrAlias]

Yeah, this was a mistake to have been merged. It was left over from other testing.