docs: Update installing-superset-from-scratch.mdx

[nytai]

SUMMARY

removing the incorrect step for building frontend assets which does not apply to the pip distro

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (494068b) 67.04% compared to head (9148f10) 69.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26468      +/-   ##
==========================================
+ Coverage   67.04%   69.16%   +2.12%     
==========================================
  Files        1948     1948              
  Lines       76062    76062              
  Branches     8493     8493              
==========================================
+ Hits        50995    52609    +1614     
+ Misses      22887    21273    -1614     
  Partials     2180     2180              

Flag	Coverage Δ
hive	53.68% <ø> (ø)
javascript	56.50% <ø> (ø)
mysql	78.06% <ø> (ø)
postgres	78.16% <ø> (ø)
presto	53.63% <ø> (?)
python	82.86% <ø> (+4.41%)	⬆️
sqlite	77.75% <ø> (ø)
unit	55.87% <ø> (?)

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[dpgaspar]

/testenv