You can only delete a document once you have a DocumentReference to it. To get that you must first execute the query, then loop over the QuerySnapshot and finally delete each DocumentSnapshot based on its ref.

var jobskill_query = db.collection('job_skills').where('job_id','==',post.job_id);
jobskill_query.get().then(function(querySnapshot) {
  querySnapshot.forEach(function(doc) {
    doc.ref.delete();
  });
});

I use batched writes for this. For example:

var jobskill_ref = db.collection('job_skills').where('job_id','==',post.job_id);
let batch = firestore.batch();

jobskill_ref
  .get()
  .then(snapshot => {
    snapshot.docs.forEach(doc => {
      batch.delete(doc.ref);
    });
    return batch.commit();
  })

ES6 async/await:

const jobskills = await store
  .collection('job_skills')
  .where('job_id', '==', post.job_id)
  .get();

const batch = store.batch();

jobskills.forEach(doc => {
  batch.delete(doc.ref);
});

await batch.commit();

//The following code will find and delete the document from firestore

const doc = await this.noteRef.where('userId', '==', userId).get();
doc.forEach(element => {
    element.ref.delete();
    console.log(`deleted: ${element.id}`);
});