bug(tooltip): matTooltip leaks memory

This issue has been reported previously, e.g. in #12644

#### Reproduction

Steps to reproduce:
1. Visit https://material.angular.io/components/tooltip/overview
2. Take memory snapshot.
3. Wiggle cursor over anything with an on-hover tooltip. (approx 100 times)
4. Take memory snapshot.
5. Repeat 3 and 4 to see an upward trend in memory usage.


#### Expected Behavior

Memory usage does not increase indefinitely due to toggling tooltip display.

#### Actual Behavior

It does.

#### Environment

- Angular: 10.0.1
- CDK/Material: Material
- Browser(s): Chrome 84.0
- Operating System (e.g. Windows, macOS, Ubuntu): gLinux

Start with the reproduction at https://material.angular.io/components/tooltip/overview and compare memory snapshots while repeatedly moving the cursor over elements with hover tooltips. Investigate the matTooltip behavior involved in toggling display, using the earlier report in #12644 as context. Done means repeated tooltip toggling no longer produces an indefinite upward trend in memory usage.

Written by the indexing model from the issue text.