Remove unused build output buffer

[rgov]

In verbose mode, output from the build command was constantly being appended to an unused buffer, character by character. Even a simple project using ESP-IDF would generate over 5 million append operations, which is quite slow because the backing memory for this buffer has to be constantly reallocated.

The actual buffer that's used is the local line variable. An alternative fix would be to remove line and use self._buffer instead, so that the get_buffer() method will return the buffer contents.

	Time to compile
Baseline (non-verbose)	22.97 seconds
Before patch (verbose)	1722.01 seconds
After patch (verbose)	24.23 seconds

Fixes #4783.

[CLAassistant]

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[ivankravets]

Thanks for the PR!