[minor] warnning: variable 'eject' set but not used

## Overview

Compiling this with the next flags:

```
  -Wall
  -Wextra
```

Output: 

```cpp
.pio/libdeps/TTGO_T7S3/MH-Z19/src/MHZ19.cpp: In member function 'void MHZ19::cleanUp(uint8_t)':
.pio/libdeps/TTGO_T7S3/MH-Z19/src/MHZ19.cpp:599:13: warning: variable 'eject' set but not used [-Wunused-but-set-variable]
        uint8_t eject = 0; 
                     ^~~~~
```