mem_drain: Remove `TIMESTAMP_FORMAT` `const` as it is used once
mem_drain: Remove `TIMESTAMP_FORMAT` `const` as it is used once

The `TIMESTAMP_FORMAT` `const` was not helping on the code
understanding and was also placed inside a method which is
confusing. Drop it.

Fixes following lint:

,----
| error: adding items after statements is confusing, since items exist from the start of the scope
|   --> src/mem_drain.rs:40:13
|    |
| 40 |             const TIMESTAMP_FORMAT: &str = "%b %d %H:%M:%S%.3f";
|    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|    |
|    = note: `-D clippy::items-after-statements` implied by `-D clippy::pedantic`
|    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#items_after_statements
`----

Signed-off-by: Otavio Salvador <1d31b11c1b92bf5d043d36edfe7174db39a6b891@ossystems.com.br>

UpdateHub/updatehub