build: remove redundant examples build

[nielspardon]

The Substrait Spark examples are already being included in the default Gradle build:

substrait-java/settings.gradle.kts

Line 14 in 631df9d

"examples:substrait-spark",

The additional examples build in Github Actions essentially also just runs the regular Gradle build besides creating a couple directories and copying files around:

substrait-java/examples/substrait-spark/justfile

Lines 18 to 30 in 631df9d

	# Builds the application into a JAR file
	buildapp:
	#!/bin/bash
	set -e -o pipefail
	${CWDIR}/../../gradlew build
	# need to let the SPARK user be able to write to the _data mount
	mkdir -p ${CWDIR}/_data && chmod g+w ${CWDIR}/_data
	mkdir -p ${CWDIR}/_apps
	cp ${CWDIR}/build/libs/substrait-spark*.jar ${CWDIR}/_apps/app.jar
	cp ${CWDIR}/src/main/resources/*.csv ${CWDIR}/_data

I think we can remove the redundant examples build step from Github Actions and shorten the build time.