Pin Loom plugin version literally for Gradle 9

Gradle 9 rejects property interpolation inside plugins {} — the version
must be a literal String. Drop the now-unused loom_version key from
gradle.properties so it does not desync from the build script.

build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'net.fabricmc.fabric-loom' version "${project.loom_version}"
+    id 'net.fabricmc.fabric-loom' version '1.17-SNAPSHOT'
     id 'maven-publish'
 }