element.py: remove call to _source_cached()

closes #1143 (closed), #1150 (closed)

tasks

• benchmark freedesktop-sdk (bst show ~4s on wsl docker build)
• benchmark debian build

on wsl

====show====
master time: 11.21
branch time: 9.89
which is a -11.78% difference
master memory: 186.0
branch memory: 185.0
which is a -0.54% difference
====build-4====
master time: 570.85
branch time: 574.51
which is a +0.64% difference
master memory: 197.0
branch memory: 197.0
which is a -0.0% difference
====build-8====
master time: 574.76
branch time: 577.03
which is a +0.39% difference
master memory: 197.0
branch memory: 197.0
which is a -0.0% difference
====build-12====
master time: 561.41
branch time: 563.32
which is a +0.34% difference
master memory: 197.0
branch memory: 197.0
which is a -0.0% difference
====show====
master time: 16.27
branch time: 9.89
which is a -39.21% difference
master memory: 201.0
branch memory: 185.0
which is a -7.96% difference