Fix/Update to remove Facebook tabs (works as of 9/28/21):
1. If you're an admin of the Facebook page, go to the following link and replace [yourpagename] with the name of your page:[yourpagename]/settings/?tab=apps 
2. The name of the tab will appear as the name of the app itself, so you'll need to click Edit Settings underneath the title of each tab to see what what each tabs' actual name is.
3. On the far right of the tab you want to remove click the x button.
Image below shows what this looks like.
If the above method doesn't work:
Since Facebook has presently (as of 9/28/21) removed the button to remove or hide a third party tab on their page interface the next best thing is:
Make a tab not appear on the front page interface:
Make 3rd party tabs less visible by moving them to the bottom of your tab order via your page settings, this will make it so that tab won't appear on the front page interface.
Do this by reordering your tabs:
Go to your Facebook page > under the page cover photo and title click More > in the drop down menu that appears click Edit Tabs > Click and drag your tab to the fifth or lower tab -- You could do this by having Facebook tabs like Home, Photos, Videos, and About take up the first four tab spots.
Directions if Remove Tab function is working on Facebook interface:
To remove a tab, on your page click on Settings (lower left side of your page, you may need to bring your mouse cursor over this section and scroll down to see the Settings button) >  click on Templates and Tabs > on the tab you want to remove, toggle the button on your tab on (button becomes blue) or off (button becomes gray). 
More Info
A Facebook bug breaking remove Facebook page tab functionality appears to be be Facebook to be fixed, but this issue doesn't discuss this functionality being removed from the facebook page interface. Bug issue here:
Some of Facebook's first party apps can be toggled on or off.
Facebook help article removing, reordering, and adding tabs here: