[nrfconnect] disable support for 2Mbps BLE PHY by default

[LuDuda]

Problem

The BLE Central implementation from (#3835) seems to violate BLE specification when nRF side initiates BLE PHY change to 2Mbps. In order to unblock progress, temporary disable 2Mbps BLE PHY by default and use standard 1Mbps. As a side effect the firmware should be smaller as well.

Summary of Changes

Disable support for BLE 2Mbps mode by default.

Fixes #3835

[github-actions]

Size increase report for "nrfconnect-example-build" from 18badf3

File	Section	File	VM
chip-lock.elf	shell_root_cmds_sections	-8	-8
chip-lock.elf	rodata	-40	-40
chip-lock.elf	text	-2440	-2440
chip-lighting.elf	shell_root_cmds_sections	-8	-8
chip-lighting.elf	rodata	-40	-36
chip-lighting.elf	text	-2440	-2440
chip-shell.elf	rodata	-40	-40
chip-shell.elf	text	-2448	-2448

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.shstrtab,0,-3
.debug_aranges,0,-8
shell_root_cmds_sections,-8,-8
rodata,-40,-40
.debug_frame,0,-44
.debug_ranges,0,-72
.debug_str,0,-81
.debug_line,0,-401
.debug_loc,0,-511
.strtab,0,-769
.debug_info,0,-779
.symtab,0,-1088
text,-2440,-2440

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.shstrtab,0,-3
.debug_aranges,0,-8
shell_root_cmds_sections,-8,-8
rodata,-36,-40
.debug_frame,0,-44
.debug_ranges,0,-72
.debug_str,0,-81
.debug_line,0,-405
.debug_loc,0,-515
.strtab,0,-769
.debug_info,0,-779
.symtab,0,-1088
text,-2440,-2440

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.shstrtab,0,1
.debug_aranges,0,-8
rodata,-40,-40
.debug_frame,0,-44
.debug_ranges,0,-72
.debug_str,0,-81
.debug_line,0,-405
.debug_loc,0,-515
.strtab,0,-769
.debug_info,0,-779
.symtab,0,-1088
text,-2448,-2448