Paging out ROM after boot up
The below circuit shows how it can be done. The principle is taken from the Z80 Second Processor for the BBC Micro and I'm sure the idea was around long before then, these things usually are.
The Z80 will use two address maps: a start-up map with the ROM available (ALLRAM is LOW) and the operating map with no ROM, just RAM (ALLRAM is HIGH).



simulate this circuit &ndash; Schematic created using CircuitLab
The start-up address map is:
ADDRESS RANGE    READ                 WRITE
0000..3FFFh      ROM image (16 KB)    RAM Low (first  16 KB)
4000..7FFFh      ROM image (16 KB)    RAM Low (second 16 KB)
8000..FFFFh      RAM High  (32 KB)    RAM High

Note that the ROM image appears twice. This harmlessly simplifies the address decoding. All writes in the 64 KB map go to the 64 KB RAM, so the RAM can be initialised easily.
The operating address map is:
ADDRESS RANGE    READ AND WRITE
0000..3FFFh      RAM Low  (32 KB)
8000..FFFFh      RAM High (32 KB)

The start-up address map is in use until the first instruction fetch from the upper 32 KB, which is always RAM.
After reset, the Z80 will execute from ROM address 0000h. Its ROM software can then follow the following example steps to get into the operating mode and be running CP/M:
Initialise the RAM with CP/M, copied from the ROM or wherever.
Store a 3-byte 'JP startCPM' instruction at 8000..8002h.
Do anything else the ROM needs to, before it vanishes until the next /RESET.
Execute a 'JP 8000h' instruction from ROM.

At the start of the JP instruction byte fetch cycle from 8000h, the D-type Flip-Flop (DFF) output ALLRAM will be preset to HIGH. ALLRAM will stay HIGH until the next /RESET.
This circuit uses OR and NOR logic gates, to illustrate the mechanisms used. It may well be possible to optimise it into fewer gates. When building one for real, though, you could look at using a CPLD instead (5 V-tolerant or 3V3, depending on your Z80 and memories) such as an XC9500XL part. A CPLD would be cheap and mop up the gates but it just depends what suits you.