Remove theme step from welcome experience

[rossmoody]

Per direction from @davidtemkin, it's time to omit the theme selection panel from the Welcome experience.

@rebron

[bsclifton]

Fixed with brave/brave-core#3242

(thanks for closing, @rossmoody! 👋😄)

[btlechowski]

Verification passed on

Brave	1.1.10 Chromium: 78.0.3904.108 (Official Build) beta (64-bit)
Revision	4b26898a39ee037623a72fcfb77279fce0e7d648-refs/branch-heads/3904@{#889}
OS	Ubuntu 18.04 LTS

Verified that theme step is removed from brave://welcome

Verification passed on

Brave	1.1.7 Chromium: 78.0.3904.108 (Official Build) beta (64-bit)
Revision	4b26898a39ee037623a72fcfb77279fce0e7d648-refs/branch-heads/3904@{#889}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified that theme step is removed from brave://welcome

Verified passed with

Brave	1.1.18 Chromium: 79.0.3945.56 (Official Build) (64-bit)
Revision	73cc6bf591f792b99f8fc7cdfb8addedbd084bf8-refs/branch-heads/3945@{#788}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified that theme step is removed from brave://welcome