fix(ios): only delete build dir when force clean build

[janvennemann]

JIRA: https://jira.appcelerator.org/browse/TIMOB-27155

Optional Description:

Fixes a regression introduced when moving from wrench to fs-extra which would delete the entire build dir on every build.

[build]

	Warnings
⚠️	iphone/cli/commands/_build.js#L5884 - iphone/cli/commands/_build.js line 5884 – Avoid calling back inside of a promise. (promise/no-callback-in-promise)
⚠️	iphone/cli/commands/_build.js#L6411 - iphone/cli/commands/build.js line 6411 – 'code' is defined but never used. Allowed unused args must match /^.+/. (no-unused-vars)
⚠️	iphone/cli/commands/_build.js#L6411 - iphone/cli/commands/build.js line 6411 – 'out' is defined but never used. Allowed unused args must match /^.+/. (no-unused-vars)

	Messages
📖	✅ All tests are passing Nice one! All 1750 tests are passing. (There are 239 tests skipped)

Generated by 🚫 dangerJS against 0f55419

[ssekhri]

FR Passed. Incremental builds working fine for iOS
Verified on:
Mac OS 10.14.3
Ti SDK: 8.2.0.v20190617024817
Appc CLI: 7.0.11
Node: 10.13.0
JDK: 1.8.0_144
XCode: 10.2.1