README: remove text about GPU mining (ethereum#26609)

Co-authored-by: Felix Lange <fjl@twurst.com>

README.md

@@ -321,12 +321,8 @@ also need to configure a miner to process transactions and create new blocks for
 
 #### Running a private miner
 
-Mining on the public Ethereum network is a complex task as it's only feasible using GPUs,
-requiring an OpenCL or CUDA enabled `ethminer` instance. For information on such a
-setup, please consult the [EtherMining subreddit](https://www.reddit.com/r/EtherMining/)
-and the [ethminer](https://github.com/ethereum-mining/ethminer) repository.
 
-In a private network setting, however, a single CPU miner instance is more than enough for
+In a private network setting a single CPU miner instance is more than enough for
 practical purposes as it can produce a stable stream of blocks at the correct intervals
 without needing heavy resources (consider running on a single thread, no need for multiple
 ones either). To start a `geth` instance for mining, run it with all your usual flags, extended