fix(internal/librarian): default to source_roots when no remove_regex

[zhumin8]

It is expected behavior to defaults to the source_roots as documented. Adding missed implementation.

librarian/internal/config/state.go

Lines 116 to 119 in 8f409cc

	// A list of regular expressions for files and directories to remove before copying generated code.
	// If not set, this defaults to the `source_roots`.
	// A more specific `preserve_regex` takes precedence.
	RemoveRegex []string `yaml:"remove_regex" json:"remove_regex"`

Fixes #1739

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (8f409cc) to head (f9da4c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1863      +/-   ##
==========================================
+ Coverage   80.53%   80.55%   +0.02%     
==========================================
  Files          75       75              
  Lines        8185     8194       +9     
==========================================
+ Hits         6592     6601       +9     
  Misses       1237     1237              
  Partials      356      356              

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