[EPM pallet]: remove `number of signed submissions` (#10945)

* [EPM pallet]: remove `number of signed submissions`

Closing #9229

* fix tests

* remove needless assert

* Update frame/election-provider-multi-phase/src/lib.rs

* cargo fmt

Signed-off-by: Niklas <[email protected]>

* fix grumbles

* cargo run --quiet --profile=production  --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_election_provider_multi_phase --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/election-provider-multi-phase/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* drop `num_signed_submissions` in WeightInfo too

* fix build

Co-authored-by: Parity Bot <[email protected]>