Remove the state rent API

[cmichi]

Closes #997.

Follow-up to paritytech/substrate#9669.

I'll remove the subsistence threshold stuff once we merge the deposit PR into Substrate.

[paritytech-ci]

🦑 📈 ink! Example Contracts ‒ Size Change Report 📉 🦑

These are the results of building the examples/* contracts from this branch with cargo-contract 0.15.0:

	Δ Original Size	Δ Optimized Size	Total Optimized Size
accumulator			6.13 K
adder			7.37 K
contract-terminate			1.50 K
contract-transfer			8.09 K
delegator	-0.01 K	-0.00 K	10.42 K
dns			22.05 K
erc1155	-0.03 K	+0.01 K	45.71 K
erc20	-0.05 K	-0.05 K	10.40 K
erc721	-0.01 K	-0.07 K	36.12 K
flipper			2.02 K
incrementer			1.92 K
multisig	-0.01 K	-0.00 K	45.83 K
rand-extension			5.35 K
subber			7.38 K
trait-erc20	+0.02 K	+0.02 K	27.48 K
trait-flipper			1.83 K
trait-incrementer	-0.15 K	-0.05 K	1.91 K

Link to the run | Last update: Wed Nov 24 15:17:03 CET 2021

[codecov-commenter]

Codecov Report

Merging #1036 (3f0c6cd) into master (3f0808b) will increase coverage by 1.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
+ Coverage   62.57%   64.12%   +1.55%     
==========================================
  Files         246      246              
  Lines        9258     9238      -20     
==========================================
+ Hits         5793     5924     +131     
+ Misses       3465     3314     -151     

Impacted Files	Coverage Δ
crates/engine/src/ext.rs	73.42% <ø> (+10.92%)	⬆️
crates/engine/src/types.rs	66.66% <ø> (ø)
crates/env/src/api.rs	36.36% <ø> (+4.54%)	⬆️
crates/env/src/backend.rs	80.64% <ø> (ø)
...tes/env/src/engine/experimental_off_chain/impls.rs	54.71% <ø> (+12.26%)	⬆️
.../env/src/engine/experimental_off_chain/test_api.rs	83.67% <ø> (ø)
crates/env/src/engine/off_chain/db/chain_spec.rs	94.59% <ø> (+18.50%)	⬆️
crates/env/src/engine/off_chain/impls.rs	42.60% <ø> (+9.56%)	⬆️
crates/env/src/engine/off_chain/mod.rs	89.47% <ø> (+11.55%)	⬆️
crates/env/src/engine/off_chain/test_api.rs	80.85% <ø> (+6.38%)	⬆️
... and 26 more

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[xgreenx]

I think you can also remove usage of this =)