Remove promote_common and common_type

[mcol]

Summary

After #1715, there are no uses of promote_common. Since this was the only user of common_type, and the latter was already planned for removal in #1243, I'm removing both of them. Fixes #1243.

Tests

Removed tests for the two functions.

Side Effects

None.

Checklist

• Math issue common_type function deprecated by return_type #1243

• Copyright holder: Marco Colombo

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.93	4.98	0.99	-1.1% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.99	-1.02% slower
eight_schools/eight_schools.stan	0.09	0.09	1.01	0.6% faster
gp_regr/gp_regr.stan	0.23	0.22	1.04	4.01% faster
irt_2pl/irt_2pl.stan	6.08	6.06	1.0	0.28% faster
performance.compilation	88.36	86.94	1.02	1.61% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.64	7.58	1.01	0.84% faster
pkpd/one_comp_mm_elim_abs.stan	21.45	21.4	1.0	0.25% faster
sir/sir.stan	96.58	94.2	1.03	2.47% faster
gp_regr/gen_gp_data.stan	0.04	0.05	0.93	-7.3% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.0	2.99	1.0	0.05% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.33	0.32	1.04	3.57% faster
arK/arK.stan	1.75	1.73	1.01	0.98% faster
arma/arma.stan	0.8	0.79	1.02	1.57% faster
garch/garch.stan	0.58	0.58	1.0	0.22% faster
Mean result: 1.00532299029

---

Jenkins Console Log
Blue Ocean
Commit hash: eff0f09

---

Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

[rok-cesnovar]

Easiest review yet! Thank @mcol